The present invention relates to a power amplifier circuit, and more particularly to a power amplifier circuit for use in audio devices.
Amplifiers for delivering output power to loudspeakers or other loads include class A and class B power amplifiers. Class A amplifiers have a pair of output amplifying devices which are in their active region throughout the entire signal cycle. Since the output amplifying devices do not operate below a cutoff bias value, the class A amplifiers suffer from no switching distortion. However, the bias currents are required irrespective of whether input signals are applied or not, and hence the class A amplifiers undergo high heat loss. Class B amplifiers, on the other hand, require a smaller bias current and are subject to reduced heat loss as compared with class A amplifiers. However, class B amplifiers suffer from switching distortion as the output amplifying devices operate alternately below the cutoff bias value.
There has been proposed a circuit arrangement designed to utilize the advantages, but eliminate the disadvantages of both class A and B amplifiers, as disclosed in U.S. Pat. No. 4,217,556.
The disclosed amplifier circuit comprises a class A push-pull amplifier circuit for signal amplification and a class B push-pull amplifier circuit for power supply amplification. The class B push-pull amplifier circuit includes a pair of driver transistors which are required to have a large dielectric strength due to the wide range of operating voltages. This prior amplifier circuit is also disadvantageous in that the negative feedback loop in the class B push-pull amplifier circuit tends to become unstable in operation, and undergoes oscillation when the parasitic capacitance in the transistors varies to a large extent. Furthermore, the high-frequency characteristics of the open loop in the class B push-pull amplifier circuit are subject to large changes due to the signal amplitudes when the cutoff frequency of the transistors is lowered as the latter are nearly saturated. Increased phase compensation in the class B push-pull amplifier circuit to avoid the above difficulties would lead to different frequency characteristics of the class A and B amplifiers circuits, causing malfunctioning of the amplifier circuit.